In order to increase the power reserve, by increasing the number of turns of a mainspring, secured at the inner end thereof to a core formed either by an arbor, which is generally cylindrical, or by a more solid boss, one solution consists in decreasing the diameter of the barrel arbor and of the associated boss, so as to increase the space available for the spring inside the drum.
The ratio of the core radius to the spring thickness is usually comprised between 10 and 20 and the invention proposes to reduce this ratio to below 10, and preferably to within a range of between 5 and 10.
The sizing must not be too small; there is a risk of breakage if the diameter of the core is too small.
In the conventional barrel architecture, a ratchet is axially mounted on a barrel arbor or on a core, via a square, with the ratchet usually being secured by an axial screw. The dimension of this screw and that of the square thus define the minimum diameter of a pivot bearing surface. A shoulder joined to this pivot bearing surface limits the axial shake of the arbor or of the core relative to a main plate or to a bar carrying a jewel or similar element.
In particular, it is not sufficient merely to reduce all of the dimensions, since the cross-sections of material are then insufficient to ensure fatigue resistance.
The issue is thus to reconcile the smallest possible diameter, to allow the largest possible power reserve, with rigidity in the ratchet drive.
US Patent Application No 804728A in the name of JOHNSON describes a barrel whose arbor carries a threaded hub, which in turn includes a mainspring. Two components are therefore necessary for the internal holding of the spring. The drum barrel is guided by a jewel which is not in a single piece with the arbor. The rewinding ratchet is located on the other side of the barrel bar. The portion carrying the square driving the ratchet is not formed by a collar, but by the end of the arbor which protrudes beyond the bar, on the opposite side to the drum.
US Patent Application No 168581A, in the name of Potter, published in 1875, discloses a barrel with a substantially smooth arbor pivoting at both ends thereof between two plates on either side of a main plate in which the barrel is embedded and which forms a cover for the drum. This arbor carries a hollow fusee carrying a wheel and onto which the inner end of the spring is hooked. In abutment on the fusee, the arbor carries the drum onto which the outer end of the spring is hooked. Next, the arbor carries the ratchet, which is in abutment on a small step of the arbor and confined underneath one of the lateral plates.
FR Patent Application No 2329000A1 in the name of ETA discloses a barrel with an arbor having a non-cylindrical shaft, the hook for hooking the spring being contained in a recess delimited by a geometrical cylinder defined by the upper and lower bearing surfaces of the shaft
CH Patent Application No 83330A in the name of BEAULIEU WATCH CO describes a barrel with no cover, including a core screwed onto an arbor. The drum is loosely fitted onto the arbor, between a bearing surface of the arbor and the core. Here too, two components are required for the internal holding of the spring.
FR Patent Application No 2210784A1 in the name of ETA discloses a barrel with an arbor comprising longitudinal grooves for hooking the spring.
CH Patent Application No 15286A in the name of DANASINO, published in 1898, discloses a barrel with a smooth arbor which carries a hollow fusee including a ratchet. This hollow fusee has two shoulders receiving the drum and the cover. The cross-section of the mainspring increases gradually from the outer end towards the inner end thereof.
US Patent Application No 1561673A in the name of ULERY, published in 1925, discloses a barrel with an arbor, and including a collar acting as support for a toothed wheel driving the drum. This arbor includes two housings at its opposite ends, one of which receives a pin for guiding the arbor in a bearing housed in a main plate and holding the end of said arbor in said bearing, and the other of which receives a pinned conical peg which has a similar bearing surface for upper guiding in a bar, and, at the distal end thereof, a ratchet drive square.
EP Patent Application No 2570864A1 in the name of BLANCPAIN discloses a barrel with a core which includes, in succession, means for securing a ratchet, and/or a first pivot bearing surface in a main plate, a drum pivot bearing surface, and collar protruding relative to said bearing surface and abutting on an inner surface of the drum towards the spring, set back radially relative to the collar, a spring receiving surface, then, set back radially relative thereto, a pivot bearing surface for a cover closing the barrel, and a second pivot bearing surface in a main plate.